The objective of this proposal is to define the viral and cellular gene(s) involved in induction of SV40-transformed cells using a temperature-sensitive mutant of SV40. The mutant can be rescued from its integrated state in transformed cells at elevated temperatures. The conditional properties of the 1501 mutant enable us to manipulate the process of induction experimentally without the complex effects of the other physical and chemical techniques previously used to induce transformed cells. We will attempt to select for other similar thermoinducible SV40 mutants. We will quantitate the induction process and characterize the uninduced 1501-transformed cell by assessing the arrangement of the virus sequences in the DNA of the transformed cells. The mutation will be mapped by the use of restriction endonucleases. An early region mutation will be confirmed by interspecies complementation. Cell hybridization with other transformed cell lines will be used to determine whether the gene product regulates viral DNA integration by positive or negative control. We will determine whether the host-cell genome plays a role in maintaining viral DNA integration. We will try to determine whether a specific excision-repair enzyme, required for induction, can be identified. SV40-transformed revertant cells will be used to examine whether integrated SV40 DNA confers resistance to supertransformation by a homoimmune virus. By delineating the mechanisms of induction, we should be able to provide a model for viral rescue from in vivo tumors.